1. Field of the Invention
The present invention relates to a method of preparing fine crystal particles of maleic anhydride, and to a maleic anhydride composition containing such fine crystal particles of maleic anhydride.
2. Related Art Statement
Maleic anhydride has been used for modifying various properties of polyolefins, including adhesiveness, dyeing properties or dyeabilities and plating properties of such resins. In the conventional process wherein a molten mass of maleic anhydride is cooled spontaneously under stationary condition, the entire molten mass is solidified and crystallized to form a large plate-shaped or lumps of crystals. However, there is a demand for maleic anhydride of fine particle form, since the fine particles are more easily handled and is improved in dispersibility and operation efficiency.
The known methods for preparing maleic anhydride of fine particle form include recrystallization of maleic anhydride from a solution in an organic solvent, such as acetone, toluene or benzene, and mechanical pulverization effected by using a feather mill (a knife hammer rotation type of screen mill) which pulverizes maleic anhydride blocks by shearing action.
Although crystals of maleic anhydride in fine particle form may be prepared by dissolving a raw maleic anhyride material in a solvent to form a solution from which maleic anhydride is recrystallized, the yield of the product is relatively low. Further disadvantages of the recrystallization method are that it involves additional steps of removing the solvent from the fine particles of maleic anhydride and recovering the solvent, and that the use of a solvent poses problems of danger and pollution of environment. Furthermore, a trace of water contained in the organic solvent used readily reacts with maleic anhydride to thereby increase the content of free maleic acid in the product.
On the other hand, when maleic anhydride having a low melting point (52.5.degree. C.) which is one of the materials sensitive to heat is pulverized through a mechanical pulverization method, it is necessary to decrease heat to be generated at pulverization step and to remove the generated heat as soon as possible. For this reason, it becomes necessary to obviate the use of pulverization mechanism which generates a large amount of heat and to provide means for remove the generated heat. As a result, a relatively expensive pulverization equipment is needed to thus cause an economical problem.
Alternatively, maleic anhydride of fine particle form readily cakes so that caking of maleic anhydride particles occurs only by storing the same for several hours. Due to caking of fine maleic anhydride particles occurred during storage and transportation thereof, the fine maleic anhydride particles prepared by the conventional method must be crushed by a wooden hammer or similar tools to separate particles before use. In order to improve operation efficiency at weighing and mixing steps, there is a demand for maleic anhydride which is kept in the form of fine particle to thus have good fluidity so that it can be used without the occurrence of disadvantageous caking phenomenon.